A known air-conditioning system includes multiple air conditioners interconnected through a three-core cable including a power source line, a communication line, and a common line. The three-core cable allows power supply and communication. Such an air-conditioning system allows communication through a current loop that is a communication scheme for transmitting signals based on changes in a communication current. In this system, for example, the communication line, the common line, and a power source circuit included in an outdoor unit of the air conditioners are connected in series to form a transmission path through which a communication current flows. Each of the air conditioners includes a transmission circuit that controls turning-on and turning-off (passage and stop) of a communication current, and a reception circuit that detects a communication current.
For a long (e.g., 100 m or more) three-core cable, noise may occur in the communication line due to the large capacitance between the lines, and interfere with normal communication. In response to this, techniques have been developed for protecting normal communication against noise. For example, Patent Literature 1 describes a technique for improving the noise resistance by installing a resistor in parallel with the light-emitting device of a reception photocoupler included in a reception circuit The resistor increases the current threshold for communication current detection. The resistance of the resistor is determined in accordance with the communication current intensity during an on-control period. The communication current intensity during the on-control period is defined using the circuit parameters such as the resistance of a resistor on the transmission path and the power source voltage of the power source circuit.